1. Field of the Invention
This invention relates in general to a digital acquisition system and specifically to a novel data acquisition system which can simultaneously acquire data at a number of "effective" sampling rates using only a single analog-to-digital converter.
2. Description of the Prior Art
It is known to convert an analog signal by passing it through an anti-aliasing filter and then to sample and hold it at a sampling rate fs and then pass the sampled signal to an analog-to-digital converter. A clock supplies an input to the sample and hold circuit and to the analog to digital converter and causes the sample and hold gate to periodically sample the input signal at a sampling rate fs. The sample signal level is held until the analog-to-digital converter converts the signal to a digital value. This sampling process can result in what is called aliasing beat frequencies on any analog signal containing frequencies which are higher than one-half of the sampling rate fs. This is the reason that an aliasing filter is used prior to the sample and hold device so as to remove frequencies higher than one-half the sample rate.
It is necessary to change the anti-aliasing filter cut-off frequency whenever the sampling rate is changed so as to prevent aliasing.
Thus, the prior art data acquisition system is not capable of operating at multiple sampling rates with a fixed anti-aliasing filter.